STEP 1 | PREPARE THE MIXTURE
Combine sugar and cornstarch in a saucepan. Gradually add milk and stir until thickened. Remove from the heat and add egg yolks. Cook for 2 minutes while stirring constantly.
STEP 2 | FINALIZE THE MIXTURE
Remove from heat. Stir in butter and lemon zest. Gently stir in lemon juice. Cover and cool completely.
STEP 3 | FREEZE
Fold in sour cream. Pour into pastry shell and refrigerate it for 2 hours.